Beyond the desert road

Pumice and ash, scoria and grit-the harsh layers of pulverised volcanic refuse that form Rangipo Dessert east of Mount Ruapehu-may offer little succour to plants, but from such unpromising materials nature has fashioned a landscape of austere and peculiar beauty.

For motorists taking State Highway 1 through the central North Hand, one of the most memorable sections is surely the stretch be­tween Turangi and Waiouru known as the Desert Road. Although Mt Ruapehu’s bulk is often concealed by cloud, the plains of rippling brown tussock which make up the volcanic plateau provide a strangely compel­ling vista. An army of power pylons stretching into the distance completes the alien aspect of this austere landscape. Yet the actual Rangipo Desert, which gives its name to this stretch of highway, is poorly known and little visited.

The desert encompasses an area of approximately 100 square kilometres between the Desert Road and Ruapehu. Of this, a little over half is Army land and the rest is part of Tongariro National Park. The Army uses its part of the desert, along with a large area of land on the eastern (Kaimanawa) side of the Desert Road, for training and exercising.

From the road, Rangipo Desert appears as a barren, stony wasteland bereft of trees. For the curious few who venture into the interior, however, a harsh beauty is revealed, particularly in the gentle light of dawn and dusk. My own exploratory journeys into this region have left two lasting impressions: the power of erosion and the tenacity of the plant life which survives here.

As would be expected of a desert, water is in short supply here. Yet it is not through lack of rain: Rangipo receives more than 2000 mm annually—twice as much as Wellington. The problem is in retaining the water. The “soil”—an unusually coarse mixture of pumice, sand, scoria and ash from past eruptions, containing next to no organic matter—has little water-holding capacity. Much of this distinctive and unprofitable substrate has been car­ried down from the summit of Ruapehu by lahars (mud slides). The shape of the upper cone of the mountain means that the face adjoining Rangipo Desert is much more prone to this sort of devastation than other slopes are.

From below, gravity sucks the water away through the porous debris. From above, the strong winds which bring moisture one day carry it off by evaporation the next. The soil can offer only limited resistance to these two relentless forces, and, along with scorching summer heat, becomes bone-dry for much of the year.

The area is frequently lashed by wind. Funnelled by the moun­tains, the prevailing north-westerlies whistle across the desert, while southerlies roaring up between the Kaimanawa Mountains and Ruapehu also touch the landscape with their chill fury.

The winds not only dry the desert but blast exposed plants, causing wind-burn at any time of the year and often freeze-burn during the cold months. Over winter, plants have also to contend with the effect of severe frosts. These frosts break apart the top layer of the soil with what is known as “frost heave.” As the moisture in the ground freezes, it forms columnar ice crystals up to 5 cm long which prise apart and splinter the earth, leaving the surface loose and ready to be carried off by wind or rain.

For many plant species, the length of time between the last frost of spring and the first frost of autumn determines the length of the summer growing season. At Whakapapa Village, on the north-west side of the mountain, there is a period of about four months largely free from frost. In the desert, even though it is at a similar altitude to Whakapapa Village (1130 m), frosts are more likely to occur year round, and the equivalent period is only three months.

Snow, which occasionally blankets the desert and closes the road, is actually less detrimental to plants than frost is, because it provides an insulating layer against intense cold and also furnishes protection from the biting wind.

In this stark environment, plant life struggles to sustain its tenuous grip on existence. And a tenuous grip it is, for the unstable soil seems always ready to take advantage of the smallest weakness in a plant’s resolve. Without a protec­rive cover of vegetation, the unconsolidated earth is easily eroded.

Water is the main sculptor in the geographical drama that is played out in the desert, with wind executing the finer touches. Water cuts through the surface, softened by frost heave, deep into underlying layers to produce a maze of miniature canyons. Wind rounds the land­scape off as it redistributes the larger particles and blows the smaller ones away. Texture in the form of long, shallow waves is also added by the wind. These waves would be barely discernible were it not for the fact that, in the process of forming them, the wind has sorted the particles according to composition, and so colour. In consequence, they show up as orange and brown bands—”tiger stripes.”

During the winter months, when a thin layer of snow may be deposited on the lee side of each wave, the undulations become more distinct. With the moist soil now much darker in colour, the tiger stripes become a zebra pattern.

In the battle between the forces of erosion and the attempts of plants to colonise and stabilise the soil, it appears that the tide has been running in favour of erosion for at least as long as people have been taking records of the area. Patches of ground stabilised by vegetation may be several metres higher than their bare sur­rounds. In some cases, surfaces that were deposited more than 10,000 years ago have been exposed.

Yet the desert has not always been so bare. Before the Taupo eruption of 186 A.D., the whole area was wooded. Charred re­mains of old trees are still being exposed by erosion from beneath the thick layers of pumice produced in that eruption.

What has happened in the years between the Taupo eruption and present times? Did the eruption fundamentally alter the microclimate in the area, allowing tussock and scrub to grow but halting further succession to mature forest? Or has the more recent human presence had a part to play?

It is clear that in the post-Taupo eruption era a stable covering of vegetation became established. Without vegetation, ash from subsequent eruptions would have been washed away within a few years of ejection. Yet such deposits are still being eroded today. Geoff Rogers, a botanist with a longstanding interest in the area, considers that before the arrival of humans, vegetation in the area could well have been a scrubby forest dominated by bog pine, mountain celery pine and Dracophylhims. Fire, ignited by light­ning strikes, would have periodically devastated plant life.

After the arrival of Polynesians, fires became more frequent. The first fires of human origin swept the area 570 years ago, and there was another series of major fires 430 years ago. There is some evidence that the rise of red tussock in Rangipo dates from this time. Early European farmers also burnt the area sometimes as often as every five years—to stimulate more palatable new growth for stock. The most recent fire was in 1988, when a Guy Fawkes skyrocket set off a blaze that spread over 5000 hectares.

Rogers believes that it was the change from occasional to frequent burning which has tipped the balance from stable bush towards the present eroding desert landscape.

[Chapter-Break]

All plants have a few basic requirements that must be met to ensure survival: sunlight, water, nutrients and a substrate. Protection from limiting factors—the drying effect of strong winds, burn-off by freezing winds, erosion of the ground they are rooted in, shading by other plants and grazing by animals and insects—is also helpful. When faced with an environment that is particularly harsh, plants need special adaptations to survive. No matter how inhospitable a landscape may appear, some plant always finds a way to live there. Even bare rock surfaces can be colonised by mosses and lichens. Among other adaptations, lichens have enzymes which can dissolve rock and supply the plant with nutrients.

In Rangipo Desert, bristle tussock and silver raoulia survive by providing stability to their unpredictable habitat. By throwing a net of stems and roots across the surface, silver raoulia manages to colonise bare ground which would otherwise be highly susceptible to erosion. Once these plants have established themselves, mounds of wind-blown sand and litter build up around them, and before long other plants use this stabilised mound to get their roots down too.

By choosing to be colonisers, plants like raoulia give up the benefits of living in a community—such as shelter from the wind—but also avoid competition. At least for a while. Although their low, flat shape offers little wind resistance, it does make them very susceptible to being shaded out by taller plants later on.

With some plants, such as eyebrights, close proximity to other plants has a more sinister implication. Eyebrights tap into the root systems of other plants and literally steal their food. Unlike most plants in the desert—which are always trying to conserve what little water there is—eyebrights go out of their way to evaporate water constantly from their leaves. Most desert plants have stomata (the small pores through which leaves respire) only on the underside of their leaves, but eyebrights have them on both sides. This adaptation, which at first glance appears contradictory, ensures that osmotic pressure is kept in favour of the parasite, thereby ensuring that the flow of nutrients is always one way—towards them! A true vegetable vampire.

Although all the native plant species in Rangipo Desert have features that help them to survive, none is so superior to the rest that it can dominate the landscape. However, this situation is now under some threat. In the 1920s, lodgepole pine (Pinus contorta) was introduced into the area from the western USA for forestry purposes. This hardy, frost-resistant species has spread rapidly through the area and well deserves its recent “class B noxious plant” rating. It has been found growing up to an altitude of 2000 m on Mt Ruapehu (nearly 500 m higher than any native tree) and at lower altitudes has a growth rate that outstrips natives by at least 200 per cent. After only five years of growth, it starts to pro­duce an abundance of seeds so light that they can be carried on the wind for up to 12 km. Although most of the original plantations have been felled and burnt, it has been estimated that sufficient seeding wild Pinta contorta re­main to turn the whole district into a pine for­est in less than 100 years.

The Department of Conservation controls contorta on public conservation land and some unoccupied Maori land around Tongariro Na­tional Park. As part of this activity, they organ­ise Abort-a Contorta field trips, on which peo­ple from interest groups (tramping clubs, For­est & Bird Society, etc.) spend their weekends pulling out pines. The Army too has spent up to $1 million a year on contorta control. How­ever, these present measures are only just keep­ing ahead of the problem.

Another introduced plant which has required control is marram grass, commonly used to sta­bilise sand dunes. It is untroubled by frost and snow, and quickly smothers native plants. Heather, introduced to the National Park by a ranger determined to make it a grouse and game shooting estate about the time of World War I, has gradually spread from the Chateau side of the park across Rangipo; it, too, is a problem.

Across the other side of the Desert Road, introduced animals are of more concern than introduced plants. The Kaimanawa wild horses (see New Zealand Geographic, Issue 1) patrol Army land south-east of Rangipo, where their grazing causes damage to rare native plants. Occasional incursions by wide-ranging bands of horses into Rangipo Desert pose no such problem, since these particular plants do not occur here.

Hares inhabit the Rangipo, but as food is scarce and they are territorial animals, their numbers remain constant. It is said that packs of the now-extinct Maori dog once roamed the whole area.

These days, the only howls and growls to be heard in the desert come from distant vehicles as we busy humans hurry past, eager to reach our destinations and comforts. Yet perhaps those journeys would be enriched if more of us paused to explore and absorb the splendid emptiness that is Rangipo.